1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus of an internal combustion engine, which has an occlusion-type NOx catalyst in an exhaust passage.
2. Description of Related Art
In recent years, a lean combustion internal combustion engine, which is run at a lean air-fuel ratio to improve fuel consumption, has been put to practical use. When this combustion internal combustion engine is run at the lean air-fuel ratio, a three-way catalyst cannot satisfactorily purify NOx (nitrogen oxide) in exhaust gas of the engine due to purifying characteristics thereof. Recently, an occlusion-type NOx catalyst has been developed that occludes NOx in the exhaust gas when the combustion engine is run at the lean air-fuel ratio, and emits and reduces the occluded NOx while the internal combustion engine is run at a stoichiometrical or rich air-fuel ratio.
This occlusion-type NOx catalyst has the characteristics of occluding NOx in the exhaust gas as nitrate (X-NO3) with oxide being excessive in the internal combustion engine, and emitting the occluded NOx with carbon monoxide (CO) being excessive to thereby reduce the NOx to nitrogen (N2) (carbonate (X-CO3) is generated at the same time). Fuel, however, includes a sulfur (S) component, which reacts on oxide to become sulfur oxide (SOx). The occlusion-type NOx catalyst occludes the SOx instead of NOx as sulfate instead of nitrate, and this lowers the purifying efficiency of the catalyst. It has been found that the SOx occluded by the catalyst is purged (S-purge) at a rich air-fuel ratio with the catalyst being maintained at a high temperature. For example, Japanese Patent Provisional Publication No. 7-217474 discloses estimating the amount of SOx occluded by the catalyst, and raising the temperature of the catalyst and temporarily making the air-fuel ratio rich when the amount of the occluded SOx exceeds a permissible amount and the catalyst temperature is lower than a predetermined temperature. This emits the SOx and restores the purifying efficiency of the occlusion-type NOx catalyst.
According to the Japanese Patent Provisional Publication No. 7-217474, however, the SOx is emitted from the occlusion-type NOx catalyst by raising the temperature of the catalyst and making the air-fuel ratio rich regardless of the running conditions of the internal combustion engine on conditions that the amount of the occluded SOx is in excess of a permissible amount and the catalyst temperature is low.
For this reason, if a vehicle is driven in a city area and the like with repeated acceleration and deceleration, the occlusion-type NOx catalyst cannot be easily maintained at a high temperature that is required for emitting the SOx. Therefore, an SOx emission means of the occlusion-type NOx catalyst has to operate for a long time although it cannot sufficiently emit the SOx. More specifically, the temperature of the occlusion-type NOx catalyst is increased even under such a condition that it is difficult to increase the temperature of the occlusion-type NOx catalyst, and thus it is necessary to make the air-fuel ratio rich and increase the temperature of the catalyst for a longer time than in the case where the vehicle is running stably with the catalyst being at a relatively high temperature. Moreover, the temperature of the catalyst must be increased to a large degree. This deteriorates the fuel consumption.
The amount of SOx occluded by the catalyst greatly changes according to a variety of factors such as the catalyst temperature, the air-fuel ratio of the exhaust gas, the concentration of sulfur in the fuel (of different types) and the running conditions of the internal combustion engine. It is therefore difficult to accurately estimate the amount of SOx occluded by the catalyst. If the SOx is emitted from the occlusion-type NOx catalyst according to the estimated amount of SOx occluded by the catalyst as is the case with the Japanese Patent Provisional Publication No. 7-217474, the exhaust gas characteristics may be deteriorated in the case where the estimated amount of SOx occluded by the catalyst cannot be estimated accurately. Moreover, if the amount of the occluded SOx is determined with an estimation error being taken into consideration so as not to deteriorate the exhaust gas characteristics, the SOx is emitted when there is no necessity of emitting the SOx. This deteriorates the fuel consumption.
It is therefore an object of the present invention to provide an exhaust gas purifying apparatus of an internal combustion engine, which enables the stable regeneration of the catalyst device.
It is therefore an object of the present invention to provide an exhaust gas purifying apparatus of an internal combustion engine, which efficiently desorbs a sulfur component from a catalyst device to thereby prevent the deterioration of the fuel consumption and enable the stable regeneration of the catalyst device.
The above object can be accomplished by providing an exhaust gas purifying apparatus of an internal combustion engine comprising: a catalyst device provided in an exhaust passage of the internal combustion engine, the catalyst device occluding a sulfur component in exhaust gas when an exhaust gas air-fuel ratio is lean at least; and regenerating means for increasing a temperature of the catalyst device to desorb the sulfur component from the catalyst device when the temperature of the catalyst device is not less than a set temperature that is determined within such a range that the occluded sulfur component can be desorbed.
Preferably, the regenerating means includes ignition timing control means for controlling an ignition timing to increase the temperature of catalyst, and air-fuel ratio control means for controlling an air-fuel ratio of the internal combustion engine; and when the temperature of the catalyst device is not less than the set temperature, the ignition timing control means retards the ignition timing and the air-fuel control means changes the air-fuel ratio in such a manner as to achieve an excessive concentration of fuel.
It is possible to provide stop means for stopping the operation of the regeneration means if the temperature of the catalyst device does not reach an optimum catalyst temperature range for desorbing the sulfur component even though a predetermined period of time passes after the operation of the regeneration means.
It is also preferable to provide frequency estimating means for estimating a frequency at which the occluded sulfur component is brought into the state of being emitted; and operation control means for controlling an operation of the regeneration means according to an output of the frequency estimating means.
In this case, the frequency estimating means preferably has desorption frequency sensing means for calculating the frequency according to a regeneration period and a running distance of the catalyst device at a specific catalyst temperature at which the sulfur component can be desorbed.
Moreover, the operation control means preferably operates the regeneration means if an output of the frequency estimating means is small and the unoperated state of the regeneration means continues for a predetermined period of time. It is possible to provide set temperature changing means for changing the set temperature according to the output of the frequency estimating means. In this case, the set temperature changing means preferably increases the set temperature as the frequency increases or has a sensor disposed in the exhaust passage in order to sense an exhaust gas temperature, and the operation control means preferably operates the regeneration means when the temperature of the catalyst device, which is estimated from an output of the sensor, exceeds the set temperature.